The present invention is directed to the art of RF broadcast transmission systems and, more particularly, to improvements in providing pre-correction of a digital information signal prior to amplification and transmission.
It is known that various communication systems require amplification of an information data stream. The data stream may be provided in a digital format and then converted to an analog signal which is then applied to a high power amplifier. Such an amplifier may distort the signal and corrections are needed.
With the recent introduction of high definition TV (HDTV) many TV broadcasters are required by the FCC (Federal Communications Commission) to transmit at relatively high power levels. To meet such high power levels with reasonable efficiency, most amplifiers are operated in Class A or Class A/B. Even with the high linearity of these amplifiers, the peak to average ratio for the digital systems and the power levels required, the amplifiers create spectral re-growth in adjacent channels due to the memory-full (linear) and non-memory-full (non-linear) distortions. A DIGITAL RF TRANSMITTER SYSTEM EMPLOYING BOTH DIGITAL PRE-CORRECTION AND ANALOG PRE-CORRECTION
The present invention is directed to the art of RF broadcast transmission systems and, more particularly, to improvements in providing pre-correction of a digital information signal prior to amplification and transmission.
It is known that various communication systems require amplification of an information data stream. The data stream may be provided in a digital format and then converted to an analog signal which is then applied to a high power amplifier. Such an amplifier may distort the signal and corrections are needed.
With the recent introduction of high definition TV (HDTV) many TV broadcasters are required by the FCC (Federal Communications Commission) to transmit at relatively high power levels. To meet such high power levels with reasonable efficiency, most amplifiers are operated in Class A or Class A/B. Even with the high linearity of these amplifiers, the peak to average ratio for the digital systems and the power levels required, the amplifiers create spectral re-growth in adjacent channels due to the memory-full (linear) and non-memory-full (non-linear) distortions.
In addition to the higher power levels required by the broadcaster, the FCC has placed stringent emission requirements on the transmission equipment. In particular, the out-of-channel emission must be maintained at extremely low levels. In order to meet the required FCC mask for emission, a combination of pre-correction and high power filters are required on the output side of the transmitter. While these filters are effective at reducing the out-of-channel emissions, they introduce group delay to the desired transmitted channel. The group delay of the high power filter causes undesirable distortions to the transmitted signal and must be corrected.
To meet the required FCC emission mask for the digital television standard, pre-correction is used to reduce the amplifiers spectral re-growth. The output from the amplifier is then filtered with a high power filter to further reduce spectral re-growth. To remove the linear or memory-full distortions introduced by the high power filter, a pre-correction network is applied prior to the transmitter.
The transmitter amplifier can be modeled as a cascade of blocks consisting of linear and non-linear blocks. For a typical amplifier these consist of a high power amplifier input network which has a filter, the amplifying device itself, and the amplifier output network which has a high power output filter. The distortion correction circuits may be arranged in an inverse or reverse order in terms of pre-correction linear and non-linear distortion correcting circuits.
The prior art includes a co-pending U.S. patent application Ser. No. 09/312,354 filed on May 14, 1999 entitled BROADCAST TRANSMISSION SYSTEM WITH DISTRIBUTED CORRECTION and which was filed in the names of Edwin R. Twitchell and Robert J. Plonka and assigned to the same assignee as the present application. That prior art is represented herein by FIG. 2 to which reference is now made. As will be described in greater detail hereinafter, the prior art of FIG. 2 includes a power amplifier having an input filter and an output filter with the filters providing linear distortion to the information signal. The information signal is taken from a data stream of digital data that is supplied to adaptive pre-correctors located upstream from the power amplifier. These adaptive pre-correctors are represented by digital adaptive linear equalizers and an adaptive non-linear corrector located in the inverse order to that of the components introducing the distortion. Other circuits are known in the prior art wherein the pre-correction is accomplished with only analog pre-corrector circuits. The present invention is directed to improvements wherein both analog and digital pre-corrector circuits are employed.
In accordance with the present invention there is provided a digital radio frequency transmitter system having an input circuit for receiving digital signals to be amplified and transmitted. The digital signals are converted by a digital-to-analog converter into analog signals. At least one radio frequency filter and a radio frequency amplifier are located in a series circuit downstream from the digital-to-analog converter and wherein the filter and the amplifier respectively introduce linear and non-linear distortions into the analog signals for transmission by the transmitter. A digital pre-correction circuit is located upstream from the digital-to-analog converter for pre-correcting the digital signals in a manner to compensate for at least some of the distortions. An analog pre-correction circuit is interposed between the digital-to-analog converter and the series circuit for pre-correcting the analog signals to additionally compensate for at least some of the distortions prior to application to the series circuit.